Four Fundamental Forces

Perhaps the first force to become manifest
in the young Universe was the weakest of all four forces: the force of gravity.
Gravity is a force of attraction between every particle of matter in the Universe and every other particle.

Yet, despite its weakness, it is the only
force that we humans are familiar with in our everyday lives. It is the force
which holds us on to the Earth. Without it none of this story would have been
possible.

Gravity holds us on the Earth

So why can we feel gravity when we cannot
detect the other, stronger forces which we will see soon?

Because the other three are either very short
range, much smaller than a human being, or their attractions cancel out their
repulsions. Gravity, on the other hand, is a long-range force and it never
cancels itself out. There is no negative gravity in the Universe today
(although there probably was negative
gravity in the very young Universe, producing the process of inflation).

Incompatible Theories

And there is another very important
difference between gravity and the other three fundamental forces. The laws
which govern gravity, expressed by Einstein's theory of General
Relativity, are completely different and incompatible from the laws of quantum mechanics which govern the three stronger forces.

Many scientists are sure that this is
because we are lacking an even more fundamental theory, one which will
eventually unite both of these theories into a single idea. The so-called "Theory of Everything". Scientists hope that
it will be discovered one day, but at present this seems a long way off.

Happily, the effect of gravity is
negligible at the scale of particles. So quantum mechanics still works well
despite its inability to describe gravity.

String Theory

One possible way to reconcile relativity with quantum mechanics
is string theory. This is the idea that particles are actually made of short
lines in space which vibrate. They are called strings. The way they vibrate
determines what kind of particle they are. In order to explain how strings
work, the theory predicts that there are about 11 extra dimensions of spacetime, curled up very small. The mathematical
basis of string theory is very complex and so far it has failed to make any
predictions, but some people think it will eventually produce the hoped-for theory of everything.